In recent years, cascading trip-off of wind farms frequently occurs, thus bringing a challenge to security of wind farms, so that a voltage control of wind farms is particularly important. If the use of wind energy is mainly realized by sending wind energy into power grids on a large scale, since wind farms are located at end of a weak grid structure and wind power fluctuates widely, when a wind farm is tripped off, the other wind farms would be affected. However, a main network and wind farms are controlled by two different automatic voltage control systems and coordination control of the two systems is lacking, thus safe thresholds for voltages of the main network and wind farms are adjusted according to experience.